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Cyber Fighters have claimed responsibility for five waves of attacks against banks since last September

An Islamic group that has claimed responsibility for several waves of attacks on major U.S. banks since last September has promised to resume its assault next week using a lot more firepower.

The group that calls itself the Cyber Fighters of Izz ad-Din al-Qassam claimed to have given a preview of what's to come in launching distributed denial of service (DDoS) attacks against the websites of a number of banks and credit unions on Monday. Targets included Bank of America, PNC Financial, Capital One Financial, Union Bank, Zions Bank, Citizens Bank, Peoples United Bank, Patelco Credit Union and University Federal Credit Union.

Which banks were struck and the extent of the disruption were not clear. However, Ronen Kenig, director of security solutions at Radware said Thursday at least seven banks suffered outages that lasted from a few minutes to several hours. He refused to name the banks.

Security researchers tend to take the group's pre-announcements of attacks on Pastebin seriously, because the attackers have generally followed through on their promises. Also, the method of attack has been consistent.

The group generally uses compromised servers in hosting companies that are often located in the U.S., Kenig said. In the latest attacks, the servers were infected with a "shell booter," which essentially enables the attackers to flood a site with bogus traffic in order to overwhelm its servers.

Maximum traffic in the latest attacks reached more than 100 gigabits per second, far exceeding the maximum of 60 to 70Gbps in previous assaults, Kenig said. The reason the attackers have achieved such traffic levels is through the use of servers instead of compromised PCs, which have far less firepower.

The hosting companies whose servers have been compromised are legitimate businesses unaware of their complicity, Kenig said. Radware has been notifying the companies.

Despite knowing about the attacks in advance, the banks and security vendors have failed to stop them not because of a lack of technology, but because of a lack of cooperation among all the affected parties, Kenig said.

"The entire security ecosystem still does not deal with these attacks as it should," Kenig said. Hampering efforts is a lack of information caused by the reluctance of banks and vendors to share data.

In 2012, Radware reported a 170% increase in DDoS attacks over the previous year, costing financial institutions $32,560 per minute of downtime. In addition, the length of time that an attack takes place has gone from hours to as long as a month.

The Cyber Fighters have claimed responsibility for five waves of attacks against banks that have occurred since last September. The last attacks occurred roughly a month ago.

Banks have sought help from the U.S. government in battling the assaults, which some defense officials have blamed on the Iranian government. Several affected banks, including PNC, SunTrust Banks and BB&T, have asked the government to stop or at least lessen the severity of the attacks.

Iran has denied any involvement. The Cyber Fighters claim the attacks will continue until YouTube removes an anti-Islam video that mocked the Prophet Muhammad. The video, called the Innocence of Muslims, sparked violent demonstrations last year in many Middle Eastern countries.